UNIX_TIMESTAMP(STR_TO_DATE('201506', "%Y%M"

Issue:
-----
When an invalid date is supplied to the UNIX_TIMESTAMP
function from STR_TO_DATE, no check is performed before
converting it to a timestamp value.

SOLUTION:
---------
Add the check_date function and only if it succeeds,
proceed to the timestamp conversion.

No warning will be returned for dates having zero in
month/date, since partial dates are allowed. UNIX_TIMESTAMP
will return only a zero for such values.

The problem has been handled in 5.6+ with WL#946.